you could get a factory NV3500, NV4500 or SM465 all behind the TBI350. parts would be easy to source as they were all factory options. but non factory, there are other options as well.
the 4500 is the best choice IMO, easy to source, 5 speed, and tough. but as mentioned above, you will most likely have to adapt a chevy master cylinder to your jeep in order to get full throw out of the clutch. though the 4500 is most likely the most expensive choice. the 3500 is not that strong, would probably work however. the 465 is beefy, but only a 4 speed. they are everywhere but being they were designed in the 60's, it shifts like a tranny from the 60's.
1998 ZJ-- lowered a touch
2005 GMC 2500HD 496 C.I.
Fat Amy-1989 K5 blazer. Linked, lifted, 38.5's on won tons. Big Girls can play in the woods too.